1. Field of the Invention
The invention relates to a magnetically inductive flow meter having a measuring section surrounded by a wall, where a medium flows through the measuring section, and the magnetically inductive flow meter also has a magnetic field generating device, an electrode arrangement and a controller/evaluator.
2. Description of the Related Art
Magnetically inductive flow meters use Faraday's induction principle to determine the flow rate of a fluid that is flowing through the flow meter. A magnetic field is generated perpendicular to the flow direction. A voltage perpendicularly to the magnetic field and to the flow direction occurs in this magnetic field as a result of loads that are transported with the fluid, where the voltage is measureable with the aid of electrodes. The measured voltage that is determined in this manner along the measuring section is proportional to a flow rate that is determined via the flow cross section.
WO 2006/050744 A1 discloses that it is necessary for flow meters of this type to maintain geometric dimensions that are to a great extent constant in order to ensure the measuring accuracy irrespective of the actual prevailing pressure of the through-flowing fluid. This resistance to pressure is achieved via a measuring tube that is formed from steel and through which the fluid flows. In order to prevent the measuring tube from disturbing the electrical and magnetic fields that penetrate the fluid in the region of a measuring section, an insert is used in the measuring tube and the insert is essentially produced from an electrically insulating rubber or synthetic material. The insert simultaneously protects the metal wall of the measuring tube against corrosion. In order to prevent the tubular insert detaching from the inner wall of the measuring tube in the case of a pressure occurring that is essentially lower than the atmospheric pressure, whereby the flow cross section would be reduced, the electrically insulating material of the measuring tube insert is reinforced at least in one measuring section by a tubular metal mesh. The specified publication is referred to in relation to further details and advantages of the known tubular insert and also the magnetically inductive flow meter that is provided with the insert.
As a result of using electrodes to determine the induced voltage, in particular if the electrodes contact the medium in a galvanic manner, a specific conductivity of the medium is required. In addition, the medium should comprise a good homogeneity, where the measuring section by which the induced voltage is determined using the two electrodes does not include any electrically non-conductive gas bubbles, solid particles or fluid drops. Inclusions of this type lead in a disadvantageous manner to an electrically non-conductive material, i.e., a noisy signal, while detecting the induced voltage and therefore to an impairment of the measuring accuracy, in particular if the inclusions are located near to the measuring electrodes.
US 2006/0174715 A1 discloses a magnetically inductive flow meter, where the parts of the electrodes that are in galvanic contact with the medium are formed from a suitably conductive polymer material having a special shielding in order to reduce signal noise.
Furthermore, different magnetically inductive flow meters are disclosed, for example in U.S. Pat. No. 8,120,370 B2, US 2010/0011877 A1 or U.S. Pat. No. 4,631,969, where capacitive electrodes that are not in direct contact with the through-flowing medium are used to measure the voltage that is perpendicular to the magnetic field and perpendicular to the flow direction. The capacitive electrodes are not in direct contact with the flowing medium. As a result, the risk of electrode corrosion or of a galvanic noise possibly occurring at the electrode surface is avoided.